This should help you get an answer to your question for your car. It is a coil spring spring rate calculator courtesy of jbl. It appears to be dead on from the tests I have given it.
Measure your spring wire diameter, the spring outside diameter and the number of coils. Enter the data in the blank fields push compute and shazam you have a spring rate.
Here is the link to the calculator =>
http://www.jblmotor.com/spring.htm
For what it is worth you might want to do a little lab experiment on a nice riding daily driver. Find a 200 lb friend, place him on the front fender of your nice riding daily driver. Measure how far the wheel well top lip settles with your friend on the car. Divide his weight (200 in this example) by the amount the fender lip settled in inches. You now have the spring rate for that wheel.
When I did this experiment I had a 4600# daily driver that was a little north of 1100# on the wheel I measured. My daily driver spring rate was about 220#/inch.
What you will probably discover is that your nice riding daily driver will also have a spring rate of about 225#/inch give or take 25 #/inch. This is no accident. This type of springing gives that comfortable around town ride on most 2 ton vehicles.
Now get yourself one of the progressive rate springs like QA-1's 200/600 progressive and you get the boulevard ride with race track handling when you woof on it.
By the way don't forget your Cobra is about a ton less massive than the nice riding daily driver so don't be squeamish about what seems to be a wimpy 200#/inch initial spring rate. The progressive will come in quickly maybe more so than you expect.
If you have access to a good suspension dynamics shop they can help you further tune the suspension including sway bar rates.
Enjoy.
Ed
Coil Spring Rate Calculator
Threaded Mode
